Since the ids that we get from Google are guaranteed to be unique, we might want to just use those as the keys.  What you're doing here is creating users with an automatically generated id and adding a non-ID field called user_id.  

To create a user with an id of user_id, you would construct the user like User(id=user_id).  You can fetch the user the way you did (using get_by_id).  You can get the ID from a user you already have using user.key.id()